The ubiquitous single board computer, the Raspberry Pi, or the RBPi, as it is fondly called by its users, is rich in General Purpose Input Output or GPIO pins. These are lined up on the board in two rows of 13 easily accessible pins, totaling 26 of which 17 are GPIO pins, the others being either power or ground pins.
GPIO pins provide a physical interface between the RBPi and the external world. Speaking plainly, these act as switches that the user can turn on or off as inputs or the RBPi can turn on or off as outputs. GPIO pins are physically arranged along the edge of the RBPi board, next to the yellow output socket for video.
To allow the RBPi to interact with the real world, you can program the pins in amazing ways. For example, there need not be a physical switch to connect inputs. Inputs can come from a signal from a device such as another computer or a sensor. Similarly, outputs can be made to do almost anything, such as sending data or signal to another device such as an LED.
One of the advantages of having an RBPi on a network is you can control devices attached to it from remote places, while collecting data from those devices. Connecting to and controlling physical devices over the Internet is exciting and a powerful feature best done by the RBPi.
However, some applications demand more input and output pins apart from the 17 that are available on the RBPi. That requires the user to expand the GPIO pins and this they can easily do by using the Quick2Wire Port Expander board. The board adds 16 more GPIO pins to the RBPi’s 17, so you can now have 33 GPIO pins with one expander board.
Additionally, you can stack more boards to have more GPIO pins. Each expander board can be preset with a configurable address via DIP switches on-board. Since eight addresses are possible, you can add eight more boards. Each board communicates to the RBPi via the I2C bus.
The Inter Integrated Circuit Communication protocol, called I2C in short, links the micro-controller or microcomputer to other micros or circuits. Another similar protocol is the Serial-Parallel Interface or the SPI. Both protocols are widely used for robotics and hobby electronics projects.
NXP (originally Philips) developed the I2C protocol. This is a very popular protocol used in several equipment including computer motherboards, monitors and TVs. Although a very flexible protocol, I2C is rather limited in its bandwidth.
Freescale (originally Motorola) developed the SPI protocol, which is much faster as compared to I2C. However, it is somewhat more complicated to use and has its own limitations.
Modern micro-controllers now support both protocols. These include the RBPi, Arduino, BeagleBone and BeagleBoard. Therefore, with I2C, you can control a host of devices, treating them as slaves and using two lines SDA and SCL. With SPI, data rates of over 10 MHz are common. Data transfer happens over three lines, one of which carries the clock and the other two communicate between the master and the slave.